North Korea is ramping up activity at its nuclear test site in the country's northeastern area, said James Hardy, the Asia Pacific specialist at IHS Jane's Defense Weekly, citing satellite imagery analysis.

"Images taken by DigitalGlobe and GeoEye in the past month show mining carts and excavation equipment present at the active tunneling site, yielding the largest amount of spoil -- or debris recovered from inside the tunnel -- around the tunnel entrance to date," Hardy said Tuesday in a comment sent to Yonhap News Agency.

He also said the full mining train, which includes an engine and several carts, was outside the tunnel, citing an April 18 GeoEye image.

"A 9 May DigitalGlobe image shows new road networks within the spoil piles, carts and a vehicle in the site operations facility," he said.

His comment came a day after South Korea, the United States and Japan warned that North Korea will risk facing more sanctions and deepening its isolation if it conducts a nuclear test.

"I think it would be a serious miscalculation and mistake if North Korea worked to engage in a nuclear test," Glyn Davies, the U.S. special envoy for North Korea policy, said Monday in Seoul after talks with his South Korean and Japanese counterparts.

"If there is further provocation such as a nuclear test, there will be swift and sure reaction by the international community," Davies said.

There has been speculation that the communist country may carry out a nuclear test to try to compensate for last month's botched rocket launch.

The long-range rocket, which Pyongyang claimed was meant to put a satellite into orbit, exploded soon after lift-off on April 13. South Korea and the U.S. said it was a cover for testing the North's ballistic missile technology.

The North has a track record of carrying out a nuclear test following a long-range missile test.

In 2006, the North conducted its first nuclear test, three months after the test-firing of its long-range Taepodong-2 rocket. The second nuclear test in 2009 came just one month after a long-range rocket launch. (Yonhap)